A photodetecting element (for example, a photodiode, a photomultiplier or the like) can output current whose magnitude corresponds to the intensity of incident light, and can detect the light intensity on the basis of the current value. Such a photodetecting element is excellent in linearity between the incident light intensity and the output current value in a broad dynamic range with respect to incident light intensity. On the other hand, it is known that the dynamic range of sensitivity of the human eye with respect to light intensity is on the order of about six digits.
Therefore, an A/D converter for inputting a current value output from a photodetecting element and subjecting the current value to A/D conversion is required to output a digital value having a large number of bits in connection with the broad dynamic range of the light intensity as described above. For example, the digital value output from the A/D converter is 20 bits although the dynamic range of light intensity is on the order of six digits. However, it is difficult to implement an A/D converter for outputting a digital value of 20 bits as described above.
In order to solve the problem as described above, an I/F converter for outputting a signal whose frequency corresponds to the magnitude of input current has been proposed (for example, see Patent Document 1). The I/F converter is supplied with a current value output from a photodetecting element, and outputs a pulse signal whose frequency corresponds to the magnitude of the current value (that is, incident light intensity to the photodetecting element). Accordingly, by counting the number of pulses per unit time in the signal output from the I/F converter, the light intensity can be achieved as a digital value in a broad dynamic range.
Furthermore, according to the photodetector having the plurality of photodetecting elements and the I/F converter, the photodetecting elements are respectively disposed at a plurality of points, whereby the light intensity detected by the photodetecting element disposed at each point can be achieved as a digital value in a broad dynamic range. Furthermore, for example, wavelength selecting filters having different transmission characteristics are disposed in front of the respective photodetecting elements, whereby the intensities of light beams having different wavelengths can be achieved as digital values in a broad dynamic range.
FIG. 14 is a diagram showing the construction of a conventional I/F converter disclosed in Patent Document 1. The I/F converter 40 disclosed in FIG. 14 is equipped with a current-voltage converting circuit 41, a transistor Tr1, current mirror circuits 42 and 43, a mirror integrating circuit 44, a comparing circuit 45 and a reference voltage source 46.
The current-voltage converting circuit 41 has an operational amplifier 41a and a feedback resistance element Rf, and inputs a current value output from the current value detecting circuit 4, converts the current value to the corresponding voltage and outputs the voltage concerned. The transistor Tr1 is supplied with the voltage output from the current-voltage converting circuit 41 at the gate terminal thereof, amplifies the input voltage logarithmically and then makes the current corresponding to the amplified voltage flow between the source terminal and the drain terminal. The current mirror circuit 42 has transistors Tr2 and Tr3, and it amplifies and outputs the current output from the transistor Tr1. The current mirror circuit 43 has transistors Tr4 and Tr5, and it amplifies and outputs the current output from the current mirror circuit 42.
The mirror integrating circuit 44 has an operational amplifier 44a and a feedback capacitor C, and inputs current output from the current mirror circuit 43, accumulates charges in the capacitor C in accordance with the input current and outputs the voltage corresponding to the amount of the accumulated charges. The comparing circuit 45 compares the magnitude of the voltage output from the mirror integrating circuit 44 with a reference voltage Vref output from a reference voltage source 46, and outputs a comparison signal representing the compared result. A switch 34 provided between the input and output terminals of the operational amplifier 44a of the mirror integrating circuit 44 inputs the comparison signal which is output from the comparing circuit 45 and passed through a buffer amplifier 33, and it is opened and closed on the basis of the comparison signal.
In the I/F converter 40, the accumulated amount of charges in the capacitor C gradually increases as current is input to the mirror integrating circuit 44, and the voltage output from the mirror integrating circuit 44 increases. Finally, when the voltage output from the mirror integrating circuit 44 exceeds the reference voltage Vref, the comparison signal output from the comparing circuit 45 is inverted, whereby the switch 34 is closed and the capacitor C is discharged. When the capacitor C is discharged, the comparison signal is inverted again, the switch 34 is opened, and the accumulation of charges in the capacitor C is resumed. As described above, the capacitor C is repetitively charged and discharged, and the comparison signal output from the comparing circuit 45 is a signal representing the repetition of charging and discharging and having the frequency corresponding to the magnitude of the input current value.
The I/F converter 40 has a transistor Tr1 having a logarithmic amplification characteristic, and thus it can enhance the linearity of the input/output relationship between the input current value and the output frequency even when the output frequency (input current value) is so high (large) that the discharge period of the capacitor C cannot be sufficiently secured when a transistor having no logarithmic amplification characteristic is used. That is, the I/F converter 40 is used to enhance the linearity of the input/output relationship with respect to the input current value in a broad dynamic range.
Patent Document 1: Japanese Published Unexamined Patent Application No. 2002-107428
However, as described later with reference to FIG. 4, it is difficult for the photodetector having the above-described conventional I/F converter to implement the high-precision conversion in a broad dynamic range for high linearity for the input/output relationship between the incident light intensity and the output frequency. In particular, when the photodetecting device has a plurality of photodetecting elements, it is important to implement higher linearity with high precision between the incident light intensity to each photodetecting element and the output frequency from the I/F converter.
The present invention has been implemented to solve the foregoing problem, and has an object to provide a photodetecting device that can implement high linearity for the input/output relationship in a broad dynamic range with high precision.